The movie starts with Jane (Meryl Streep) and Jake (Alec Baldwin) at the anniversary party of a couple. You see the four laughing together and Jane and Jake smiling, and they seem to be at ease with each other. Everyone is of similar age and elegantly and tastefully dressed, and then Jake’s much younger wife Agness (Lake Bell) shows up wearing a bikini top and a sarong skirt. They chat for a few minutes and Agness brings up the plans for Luke’s, Jake and Jane’s son, graduation in New York the following week. They realize they are all staying at the same hotel. Jane excuses herself from the party and drives to her beautiful home. We see that she has a loving, close relationship with her two adult daughters, Gabby and Lauren, as well as Lauren’s fiancée Harley (John Krasinski). She also has a close-knit, group of friends who go over to her house for dinner and drinks, to hang out, gossip and have a good time. They discuss Jane’s amicable relationship with Jake and how much she has progressed from hating him 10 years ago to peacefully co-existing with him. They discuss how Jake cheated on Jane during their marriage with Agness, eventually leaving Jane to be with Agness. However, then Agness left Jake for a former boyfriend, got pregnant by him and returned to Jake. Consequently, Jake is now raising Agness’ young son with a former lover, Pedro.

Later, Jane goes to work, she owns and runs a bakery/café. She also realizes that she had forgotten about an appointment with her architect as she’s planning to remodel her kitchen and add an expansion to her home. Her architect brings along another architect, Adam (Steve Martin), and as they go over the plans, which Jane loves, he reveals that Adam is the one who has designed her new plans. Jane is very involved in the process, sending numerous emails concerned with minute details and has even more comments and details she wants altered. She and Adam make another appointment for Adam to come over to the house to see it in person and discuss the plans.

Jane sneaks away, telling her employees she’s going to a dentist appointment when she really goes to see a plastic surgeon. She expresses her reservations about plastic surgery but admits some concern over her drooping eyelids. The doctor details cosmetic procedures, which sound pretty grotesque and she quickly leaves. In the elevator, she runs into Jake, Agness and Pedro coming out of a fertility clinic. She offers her dentist excuse again, and Jake questions her, knowing she’s been going to the same dentist for years, further illustrating their closeness and history. We see that Pedro is hyperactive and constantly climbing all over Jake, further illustrating the age differences in his new family.

Jane arrives in New York for her son’s graduation and goes to check in at her hotel. Her children all meet her at the hotel and to her dismay, her son has planned a graduation party which all the children will attend, leaving her by herself for the evening though she had made dinner reservations for all of them. She ends up going to the hotel bar by herself and seeing Jake, also alone. Agness’ son Pedro became ill so she stayed home with him rather than go to New York. He persuades Jane to dine together, at the bar at his insistence. They have a good time, going through many bottles of wine, reminiscing and dancing, and they end up in bed together. Jake is ecstatic and marvels at how good the sex was but Jane is so drunk, and possibly disgusted with herself, that she throws up.

The family all sits together at Luke’s graduation, and they all go out for lunch afterwards. The kids marvel at how well Jake and Jane seem to be getting along and how it feels like the old times, as Jake tries to play footsie with Jane under the table, which she rejects.

After returning home, Jane is out for a jog when Adam drives up for their appointment. He is listening to a self-help tape for those going through divorce and he bitterly bemoans his ex. He sees Jane jogging past and realizes she has forgotten their appointment again, but she sees him drive up, realizes it and turns back around to the house. He shows her the revised plan and is impressed by her house as is. Jake drives up to the house to talk to Jane about their New York tryst, and Adam leaves after making another appointment with Jane. He observes Jake and Jane’s closeness as Jake puts his arm around her and they talk closely. Jane expresses her guilt and regret, but Jake says they should enjoy themselves and let the affair continue. She continues to agonize over it, confessing to her friends who encourage her to get her revenge on Agness by becoming “the other woman.” She also sees her therapist because she is wracked with guilt and tells him she isn’t sure if it’s just out of loneliness or spite or if she truly wants to get back together with her ex husband. Her therapist encourages her to explore it if she wants and not to worry about the guilt. She immediately calls Jake and asks her to meet him at a fancy hotel for a quickie. Unbeknownst to them, Lauren and Harley are eating lunch at the same hotel and considering it as a wedding venue. Harley sees them both enter the hotel and get into an elevator together, but Lauren doesn’t see it, largely as Harley keeps trying to distract her, including kissing her with his mouth full of food. Neither Jake nor Jane sees Harley or Lauren. Harley then sees a doctor rush up, as Jake began having pains right before sleeping with Jane. The doctor goes up and he explains to Jake that the medications he’s taking for his cholesterol and prostate are affecting him as he tries to exert himself too much. Hurley is relieved when the doctor returns and tells the clerk that everything is all right, but he doesn’t reveal what he saw to Lauren.

Adam goes to Jane’s house to discuss remodeling plans. He has fenced off areas of her yard where they will build the extension and setup a pair of ladders so she can see what the view from her new bedroom will be. They clearly have a good time together as she cooks him a meal and tells him about her time learning living in Paris. He tells her about his divorce three year prior when his wife left him for his best friend. He also tells Jane he admires the good relationship she seems to have with her ex. Jake sneaks over to the house and watches them through the window. He grows jealous as he watches them together.

Now that Jane is embracing their affair, she plans a romantic dinner for herself and Jake, cooking all his favorite foods, for a night when Agness has dinner plans. She even turns down a date with Adam to a French film festival so she can keep this night free for Jake. However, Agness cancels her dinner because she is ovulating and wants to conceive with Jake. Jake is unable to call Jane and let her know because Agness is watching him and questions him when he says he’s going to the bathroom and then tries to take his phone with him. Consequently, Jane is left feeling humiliated and abandoned. The next day, the whole family gathers at Jane’s house, as Luke is moving back home. Jake picks Luke up from the airport and drives him over to the house, hoping to speak with Jane. They argue and she insists that it’s over between them. Hurley appears and indicates to them that he knows what’s going on, but Jane assures him there’s nothing to worry about. She also announces that she is bringing a date to the graduation party that Hurley and Lauren are throwing for Luke.

In preparation for her date and the party, Jane ends up smoking a joint that Jake had given her earlier, intending it to be shared between the two of them. Adam arrives to pick her up, presenting her a gift of an appointment book so she won’t keep forgetting their appointments. Adam notices that Jane is acting a bit goofy and she finally confesses that she’s high. When they arrive at the party, Adam smokes the joint outside, and they’re both silly and goofy inside the party, having a great time together. Jake confronts Jane in the bathroom and says they were supposed to smoke the joint together, not for her and Adam to. She gives him the rest of the joint, and Hurley walks into the bathroom, trying to make sure nothing is going on. Jake insists that Hurley smoke with them and they all get high in the bathroom. They are all stoned and start dancing together when Agness notices the way Jake keeps holding Jane and gazing at her. Jane leaves the party with Adam and he says that he’s hungry so she takes him to her café and says she can make him anything on the menu. He wants a chocolate croissant so she shows him how to make them and makes a fresh batch just for him. They talk and have a great time together. Adam also tells her that he got the impression that Jane was seeing someone when they first met. Jane said she was but that it was over and he had nothing to worry about.

Later on, Jake shows up at Jane’s house to announce that he has left Agness, but Jane questions if he has truly left her or if Agness kicked him out. Jake brushes off the questions and tries to guilt Jane into letting him stay at her house. The children also ask Jane so she gives in and lets him stay there. He continues to try to be with her, kissing her in her garden, which she rejects. They have family movie nights, and Jake expresses how happy he is to be back with his family. One evening, Jake and the children gather in the kitchen and eat leftovers out of the fridge. Jane however is sitting on her bed, having a web cam date with Adam as he tries on old clothes and she tells him whether to keep them or not. They take a break to go to the bathroom or something, and during this time, Jake sneaks in, disrobes and lays on Jane’s bed to surprise her. He uses her laptop to block his privates, so Adam gets quite a surprise when he returns to his computer. Jane is surprised to find him there too and in the commotion, the children all come rushing in as well. They’re shocked and confused as Jack and Jane confess that they had been having an affair. Adam also listens in on his web cam. The children ask if they’re getting back together to which Jane answers no. Upset by the evening’s events, Luke and Gabby go to Lauren and Hurley’s home for the night, driving away angrily. Jake also leaves as he sees Jane is not willing to reconcile with him.

The next day, Jane drives to Lauren and Hurley’s home to speak with her children. They feel betrayed and confused and upset over their parents’ tumultuous relationship. She apologizes and tells them about her own confusion and mistakes after which they all hug and move on. She also goes to Adam’s office to apologize for what happened, but he rebuffs her, telling her that he’s going to ask his supervisor to assign a different architect for her home remodeling. She is hurt and disappointed by this but goes home.

The renovations are about to begin on a particularly rainy day, so Jane goes out to see the crew to invite them inside for their planning meeting when Adam appears, much to Jane’s happiness. They all go inside the house as Adam tells Jane that he’s been craving a chocolate croissant and asks her to make one for him.

*CUT TO THE CHASE*
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Jane (Meryl Streep) and Jake (Alec Baldwin) are amicably divorced exes even though Jake married the woman he cheated on Jane with. After a drunken tryst at their son’s graduation, they begin an affair though Jane continually feels guilty. She finally breaks it off and starts to see her architect, Adam (Steve Martin) though Jake tries to continue their affair. Eventually all is revealed to both Adam and their children who all feel betrayed. Jane decides she does not want to reconcile with Jake and tries to get Adam back, who initially turns her down but eventually returns to continue with the remodeling plans for Jane’s home and to pursue their relationship.